1. Field of the Invention
The invention concerns a solid-state switch with integral protection for connecting a load to an electrical power supply and including at least one isolated gate bipolar type transistor with an emitter-collector space through which a unidirectional current flows from the supply to the load, the transistor having (taking the emitter potential as the origin) an emitter-collector current which is a function of the gate voltage and beyond a saturation voltage substantially independent of the collector voltage and interrupter means connected to the gate responsive to an electrical quantity derived from the current flowing through the switch and adapted to apply a turn-off bias to the gate in response to the current exceeding a specific value.
2. Description of the Prior Art
As compared with conventional electromagnetically controlled switches with mechanical interruption of the current, solid-state switches have the advantages that they do not include any moving parts, which makes them silent, fast and virtually wear-free, and do not generate sparks when they operate; also, with an alternating current supply it is a simple matter to synchronize them to the supply in order to close the switch at zero voltage and to open it at zero current, which significantly reduces radiated electromagnetic interference. On the other hand, the semiconductor devices of solid-state switches are usually less able to withstand overcurrents, because of internal voltage drops in particular, and blocking the current often becomes increasingly difficult as the current to be interrupted increases, so that they are not well suited to interrupting high overcurrents. Finally, the characteristics of such semiconductor devices are usually highly dependent on their internal temperature which can cause problems with adjusting the switching thresholds.
The technician is faced with various types of semiconductor device which can operate in a controlled manner on a current flowing through them. These include thyristors and triacs, GTO thyristors, MOS transistors, conventional bipolar transistors and isolated gate bipolar transistors.
Thyristors and triacs have a good resistance to overcurrents and low internal voltage drops when conducting. However, they have the major drawback that they cannot be turned off; they cease to conduct only after the current through them is interrupted. They can be used only with alternating current supplies, and cannot achieve fast switching, especially if the load or the supply includes any reactive components. These a devices also have a restoration time after passing current during which application of a voltage is likely to turn them on again. This restoration time, associated with the neutralization of charges generated by the flow of the current (avalanche current), increases with the current flowing in the moments before cancellation. These devices are therefore ill suited to applications with integral protection.
The resistance to overcurrents may be severely compromised if the switching time is relatively long, thermal energy losses increasing with the switching time.
GTO thyristors can be turned off by injecting charges into the trigger but in practise are no better suited to integral protection as the quantity of charge to be injected into the trigger increases in proportion to the current to be interrupted, to a first approximation. Integral protection will then fail in the event of a high overcurrent, just when it is of particular utility.
MOS transistors can be turned off with minimum control energy input and, in the saturated (on) state, have a low source-drain voltage which is also approximately proportional to the current through them, so that the source-drain space of the MOS transistor can be used as an overcurrent sensor at the input of interrupter means. However, although MOS transistors withstand overcurrents well, all the more so in that the switching time can be very short, their nominal current density is limited with the result that the switches would be bulky if the power passing through them were not low. Also, MOS transistors are the most costly of the semiconductor devices mentioned above, for the same nominal power rating.
The conventional bipolar transistor is compact and low in cost; it is easy to turn off. However, it is only moderately resistant to overcurrents, which would require it to be uprated. The control power needed to saturate it (turn it on) is relatively high if a low internal emitter-collector voltage drop is required, to reduce losses in normal operation; however, the power required to turn it off in response to an overcurrent is then not negligible. The low voltage drop between the emitter and the collector can hardly be used to detect overcurrents as it varies significantly with temperature.
The isolated gate bipolar transistor has most of the advantages of a base-controlled bipolar transistor, including its low cost and small size. Because it is controlled by way of its isolated gate, it shares with the MOS transistor the advantages of resistance to overcurrents and of simplicity of control, the gate absorbing energy only during switching. However, it cannot withstand high reverse voltages, which means that precautions are necessary if the load and the supply are such that switching is likely to generate reverse voltages (anti-parallel diodes with appropriate characteristics).
In this regard, all controlled semiconductor devices except triacs are one-way conduction devices and cannot be used with full-wave alternating current electrical power supplies except in appropriate circuits, and then either in pairs whereby each device operates on one half-wave or in a circuit in which the device has the current flow through it in the same direction for both half-waves.
However, the isolated gate bipolar transistor does not have the MOS property which favors the use of integral protection, namely the fact that the internal voltage drop is substantially proportional to the current flowing through the source-drain space. Taking the emitter potential as the origin, the emitter-collector current is a function of the gate voltage and, beyond a saturation voltage, substantially independent of the collector voltage.
It is easy to see that the collector voltage is not an appropriate image of the current through the emitter-collector space because of the dependence of the gate voltage. Also, the saturation voltage varies with the temperature of the semiconductor.
The use of a series impedance in the switch to derive an interrupter means control voltage would have the drawback of increasing the voltage drop in the switch and increasing the overall size of the device.
The problem to be solved by the invention is to produce a solid-state switch with integral protection including an isolated gate bipolar transistor in which the quantity derived from the current flowing through the switch is sampled at the transistor itself and enables the specific value beyond which turn-off occurs to be appropriately representative of the overcurrent threshold at which interruption should occur.